At Prime Minister’s Questions, Ed Miliband made a mathematical error. Thankfully, he corrected himself straight away.

The subject was planning law. “Mr Speaker,” he said, “there are sites all over the country with capacity for a quarter of a million—”

No sooner had the words left his lips than he realised his mistake. “I’m sorry,” he said. “Two hundred and fifty thousand houses.”

Older readers – taught decades ago using outdated textbooks under a long-discredited education system – may not immediately recognise the problem. In their day, remarkable as it may now seem, a quarter of a million and 250,000 were generally considered to be similar figures. Indeed, in the view of some leading mathematicians, the two were more or less identical.

But times change, and 250,000 is no longer what it was. Since the credit crunch, the recession and the loss of Britain’s AAA credit rating, the value of 250,000 has declined rapidly. It is now just over an eighth of a million, and decreasing by the day.

It was a strange PMQs. David Cameron mocked his opponents unsparingly, as Tory MPs bellowed their delight; Ed Miliband, by contrast, refrained from insults, while Labour MPs glared stonily. Mr Miliband’s aim is presumably to show that a British statesman can be dignified and serious, and that PMQs need not be a puerile embarrassment to the electorate.

It’s a nice dream. The trouble, though, is that it takes two to tango. At the moment, Mr Miliband is tangoing on his own, while Mr Cameron pelts him with vol-au-vents from the edge of the dancefloor and laughs.

Also, as the main purpose of PMQs, from a leader’s point of view, is to persuade his backbenchers that he’s not the floundering dud they thought he was, an attempt to be dignified is liable to end in humiliation.

Everyone on the Labour side was naturally much too serious and statesmanlike to make a joke about Francois Hollande, so the duty fell to a Tory, Andrew Bridgen (NW Leicestershire).

“The Leader of the Opposition,” he began, “once said that ‘what Hollande is doing in France, I want to do in Britain’…”

The eruption of cheers from the Tory side meant I couldn’t make out the rest of the question, but I suspect I’d already heard the best of it.